Stabilized zirconia with up to about 35 w/o tungsten has been suggested for use in rockets in U.S. Pat. No. 3,410,716. Stabilized zirconia has also been used as a protective coating in U.S. Pat. No. 3,719,519. The patentee is concerned with matching the thermal expansions of oxide, underlayer, and substrate. Normally the underlayer consisted of either an alloy with desired thermal expansion coefficient or a mixture of the oxide and alloy.
U.S. Pat. No. 3,091,548 points out that refractory materials coated by prior art techniques have generally failed by chipping and spalling due to hard bond strength between the coating and the base plate as well as to the poor thermal shock resistance of the coating. Failure is also caused by the difference in expansion coefficients between the coating and the base material.
Recently, a NiCrAlY bond coating with a yttria-stabilized zirconia thermal barrier layer was used successfully. This system is significantly better than any previous two layer thermal barrier coating system as pointed out in U.S. Pat. No. 4,055,705. The performance of a two-layer thermal barrier system is very sensitive to the concentration of yttrium in the bond coating and the concentration of yttria in the zirconia layer. It has been shown that the performance of a two-layer thermal barrier system is also very dependent on the concentration of chromium and aluminum in the NiCrAlY bond coating.
The Ni-20 w/o Cr bond coating with a graded Ni-20 w/o Cr/ZrO.sub.2 --CaO barrier coating exhibits only modest adherence and frequently suffers from partial or fatal barrier spallation or thermal shock cracking after short periods of exposure to high temperatures. In addition, the quality control problems involved in applying a continuous graded barrier are large and tedious. Also, a Ni-20 w/o Cr bond coating coupled with calciastbilized zirconia fails in less than 100 one hour cycles between 1800.degree. and 540.degree. F.
Stabilized zirconia mixed with up to 35 w/o tungsten will not withstand many exposures at high temperatures without cracking and spalling as pointed out in U.S. Pat. No. 3,410,716. The coatings discussed in U.S. Pat. No. 3,719,519 were exposed to very few, long time period cycles below 1800.degree. F. Furthermore, the underlayer was such that the thermal expansion coefficient value was between the thermal expansion values of the substrate and the surface oxide. Such underlayer coatings consist of one or more layers depending on whether a metal, alloy, or a mixture of metal-oxide is used. Thus, matching of thermal expansion coefficients of various layers is very critical to the performance of the thermal barrier system. Furthermore, as pointed out in U.S. Pat. No. 3,091,548 a two-layer thermal barrier system fails because of the difference in expansion coefficients between the coating and the base material. Attempts to use a metal undercoat under a refractory coating have been either totally unsuccessful or have shown only limited success.
The thermal barrier coating system described in the Applicant's U.S. Pat. No. 4,055,705 comprises a NiCrAlY bond coating and a yttria- or a magnesia-stabilized zirconia thermal barrier layer. It was discovered that the performance of this two-layer thermal barrier system is very sensitive to the concentration of yttrium, chromium, and aluminum in the NiCrAlY bond coating as well as the yttria concentration in the zirconia thermal barrier layer. Thus, a Ni-25w/o Cr-6w/o Al-0.3 w/o Y bond coating is about 21/2 times better than a Ni-16.4 w/o Cr-5.1 w/o Al-0.15 w/o Y bond coating and about five times better than a Ni-16.6 w/o Cr-5.5 w/o Al-1.08 w/o Y bond coating. It has also been found that a Ni-16w/o Cr-6w/o Al-0.3 w/o Y bond coating is many times superior to a coating of Co-16w/o Cr-16w/o Al-0.3w/o Y or Fe-16w/o Cr-6w/o Al-0.3w/o Y.